Fatty acids are important components of lipids such as phospholipids and triacylglycerols. Fatty acids containing two or more unsaturated bonds are collectively referred to as polyunsaturated fatty acids (PUFA) and are known to include arachidonic acid, dihomo-γ-linolenic acid, eicosapentaenoic acid and docosahexaenoic acid. Various physiological activities have been reported for these fatty acids (Non-patent Document 1).
Among them, arachidonic acid receives attention as an intermediate metabolite convertible into prostaglandin, leukotriene and so on, and many attempts have been made to apply arachidonic acid as a material for functional foods and pharmaceuticals. Moreover, arachidonic acid is found in mother's milk and is important for infant growth, particularly for height and brain development in fetuses. For this reason, arachidonic acid also receives attention as an ingredient necessary for infant growth from the nutritional point of view, as in the case of DHA (docosahexaenoic acid).
These polyunsaturated fatty acids are expected to have applications in various fields, but some of them cannot be synthesized in the animal body. Thus, microbial techniques have been developed for obtaining polyunsaturated fatty acids by culturing various microorganisms. Other attempts have also been made to produce polyunsaturated fatty acids in plants. In these cases, polyunsaturated fatty acids are known to be accumulated, for example, as components of storage lipids such as triacylglycerols within microorganism cells or plant seeds.
More specifically, triacylglycerols are produced in vivo as follows. Namely, glycerol-3-phosphate is acylated by glycerol-3-phosphate acyltransferase to form lysophosphatidic acid, which is then acylated further by lysophosphatidic acid acyltransferase to form phosphatidic acid. This phosphatidic acid is, in turn, dephosphorylated by phosphatidic acid phosphatase to form diacylglycerol, which is then acylated by diacylglycerol acyltransferase to form triacylglycerol. Other enzymes such as acylCoA:cholesterol acyltransferase and lysophosphatidylcholine acyltransferase are also known to be indirectly involved in biosynthesis of triacylglycerols.
As described above, the reaction in which lysophosphatidic acid (hereinafter also referred to as “LPA” or “1-acylglycerol-3-phosphate”) is acylated to generate phosphatidic acid (hereinafter also referred to as “PA” or “1,2-diacyl-sn-glycerol-3-phosphate”) is known to be mediated by lysophosphatidic acid acyltransferase (hereinafter also referred to as “LPAAT”).
This LPAAT is also known as 1-acylglycerol-3-phosphate acyltransferase (E.C. 2.3.1.51). LPAAT genes have been reported so far in several organisms. As an LPAAT gene from Escherichia coli, the plsC gene has been cloned (Non-patent Document 2). In fungi, the SLC1 gene from Saccharomyces cerevisiae has been cloned (Non-patent Document 3). Likewise, LPAAT genes have also been cloned from animals and plants (Patent Document 1).
With respect to LPAAT in a lipid-producing fungus, Mortierella alpina (hereinafter also referred to as “M. alpina”), there is a report showing that the microsomal fraction of this fungus has the activity of lysophosphatidic acid acyltransferase (Non-patent Document 4). Moreover, two homologs have been reported for the LPAAT gene from M. alpina (Patent Documents 2 and 3).    Patent Document 1: International Patent Publication No. WO2004/076617    Patent Document 2: US Patent Publication No. 2006/174376    Patent Document 3: US Patent Publication No. 2006/0094090    Non-patent Document 1: Lipids, 39, 1147 (2004)    Non-patent Document 2: Mol. Gen. Genet., 232, 295-303, 1992    Non-patent Document 3: J.B.C., 268, 22156-22163, 1993    Non-patent Document 4: Biochemical Society Transactions, 28, 707-709, 2000    Non-patent Document 5: J. Bacteriology, 180, 1425-1430, 1998    Non-patent Document 6: J. Bacteriology, 173, 2026-2034, 1991